Efficient Whole-cell Catalysis for γ-Aminobutyric Acid Production Using Engineered Escherichia coli

Abstract

Abstract Background γ-Aminobutyric acid (GABA) is a non-proteinogenic amino acid that has extensive applications in the food, feed, pharmaceutical, and chemical synthesis fields. The utilization of engineered Escherichia coli in whole-cell catalysis offers a promising approach for GABA synthesis based on the rapid reaction kinetics and reduced byproduct formation. Previously, we constructed a recombinant E. coli that efficiently converts L-glutamate (L-Glu) to GABA; however, freezing and thawing of the strain and the addition of external pyridoxal 5′-phosphate (PLP) were required. The objective of this investigation was to enhance the efficiency of GABA synthesis through E. coli genetic modifications to achieve a more streamlined production process. Results First, the inducible expression conditions of the gad gene were optimized to 37°C for 6 h. Next, cell permeability was improved by overexpressing sulA in E. coli, which eliminated the need for the freeze-thaw treatment during GABA production. The overexpression of pdxS and pdxT from Bacillus subtilis strain 168 resulted in an ideal engineered strain without the addition of external PLP. Thus, an efficient whole-cell biocatalytic process was optimized. The ideal isopropyl β-D-thiogalactopyranoside concentration, cellular density, and reaction temperature were 0.2 mmol/L, 15 units, and 37°C, respectively, and the substrate consisted of a 4:1 ratio of L-glutamic acid (L-Glu) to L-monosodium glutamate (L-MSG). Ultimately, the optimized conditions were employed for a bioconversion procedure using whole cells in a 3 L bioreactor. The microbial strain was capable of being utilized for a minimum of two cycles with 1 mol/L substrate mixtures, thus achieving a GABA productivity of 103.1 g/L/h and a molar yield of 100.0%. Conclusion A whole-cell catalyst for highly efficient GABA production from a mixture of L-Glu and L-MSG was constructed by engineering E. coli, and the freeze-thaw steps and external PLP addition were not required. This research illustrates that the recently engineered strain of E. coli exhibits promise for utilization in the large-scale industrial synthesis of GABA.